Cognitive science suggests that humans have distinct mechanism for short-term memory (STM) and long-term memory (LTM). The STM is memory is associated with recalling events that last from seconds to up to a few minutes, and is also called working memory. Conversely, the LTM is memory associated with recalling events from hours up to years. The LTM is typically divided into two major subclasses: declarative memory and procedural memory. The declaratory memory is all memories that are consciously available. On the other hand, the procedural memory (also known as implicit memory) is the long-term memory of skills and procedures, or “how to” knowledge.
The declaratory memory is further divided into two categories: semantic memory, and episodic memory. The semantic memory refers to the memory of meanings, understandings, and other concept-based knowledge unrelated to specific experiences. The episodic memory refers to the memory of events, times, places, associated emotions, and other conception-based knowledge in relation to an experience.
In humans, the STM stores current memory that effectively states what is currently being experienced. This experience is then transformed into something that has happened in the past in the form of the episodic memory. The semantic memory supplants the episodic memory by converting the experiences and storing them in semantic forms. The semantic memory also allows humans to learn facts from generalized information (for example, by reading books).
Humans use such different types of memory to perform various tasks. For example, performing a simple task such as pouring a glass of milk from a milk carton requires humans to use the semantic memory to recognize a glass or a milk carton, the episodic memory to recall sequences of interactions with the glass and milk carton, and procedural memory to manipulate the glass and the milk carton with hands.
Although the distinct human memory has been studied extensively, its full mechanism and interactions between the different human memory are not yet known and remain to be discovered. For example, R. C. Atkinson et al., “Human memory: A proposed system and its control process,” The Psychology of Learning and Motivation, vol. 2, pp. 89-195, Academic Press, 1968, and John McCarthy et al, “Some philosophical problems from the standpoint of artificial intelligence,” Machine Intelligence 4, pp. 463-502, Edinburgh University Press, 1969 describes the STM being a temporary storage for information before being transferred to the LTM for long term storage.
In traditional applications of artificial intelligence (AI) logic, different knowledge base architectures are used to implement different types of memory. The STM, the episodic memory, and the semantic memory store and retrieve information using different mechanisms. Therefore, in traditional applications, each type of memory is implemented using distinct memory subsystems. Integrating different types of memory subsystem to work together, however, proved to be a challenge to AI system builders.
A theory regarding the interrelationship between different memory subsystems was discussed, for example, in R. C. Schank, “Dynamic Memory: A theory of reminding and learning in computers,” Cambridge University Press, 1982. Schank discloses storing experiences in Thematic Organization Points (TOPs) and other knowledge in Memory Organization Packets (MOPs). The MOPs and the TOPs form a script that is a sequence of English-like statements that describe various relationships between data. The data stored in the scripts may later be retrieved through search methods. Although Shank discusses the memory system functioning as episodic memory, such memory system has not been implemented, and remains theoretical.